The use of microporous multi-layered membranes as battery separators is known. See, for example, U.S. Pat. Nos. 5,480,745; 5,691,047; 5,667,911; 5,691,077; and 5,952,120.
U.S. Pat. No. 5,480,745 discloses forming the multi-layered film by co-extruding the multi-layered precursor or by heat-welding, at 152° C., pre-formed precursor layers. The multi-layered precursor, formed by either technique, is then made microporous by annealing and stretching. There is no mention of stacking precursors for the step of forming the micropores.
U.S. Pat. No. 5,691,047 discloses forming the multi-layered film by co-extruding the multi-layered precursor or by uniting, under heat (120-140° C.) and pressure (1-3 kg/cm2), three or more precursor layers. The precursor formed under heat and pressure, at a speed of 0.5 to 8 m/min (1.6-26.2 ft/min), has a peel strength in the range of 3 to 60 g/15 mm (0.2-4 g/mm). In the examples, one 34 μ separator has a peel strength of 1 g/mm and the other, about 0.5 g/mm. The multi-layered precursor, formed by either technique, is then made microporous by annealing and stretching. There is no mention of stacking precursors for the step of forming the micropores.
U.S. Pat. No. 5,667,911 discloses forming the multi-layered film by uniting (by heat and pressure or by adhesives) cross-plied microporous films to form a multi-layered microporous film. The microporous films are laminated together using heat (110° C.-140° C.) and pressure (300-450 psi) and at line speeds of 15-50 ft/min (4.6-15.2 m/min).
U.S. Pat. No. 5,691,077 discloses forming the multi-layered film by uniting, by heat and pressure (calendering), or by adhesives, or by pattern welding, microporous films to form a multi-layered microporous film. Calendering is performed at 125° C. to 130° C. for a residence time of 2 to 10 minutes. Four (4) stacked multi-layered microporous precursors are calendering between a single nip roll.
U.S. Pat. No. 5,952,120 discloses forming the multi-layered film by extruding nonporous precursors, bonding together nonporous precursors, annealing the bonded, nonporous precursors, and stretching the bonded, nonporous precursors to form a multi-layered microporous film. At least four (4) tri-layer precursors are simultaneously passed through the steps of bonding, annealing, and stretching. Bonding was performed between nip rollers at 128° C. (range 125° C.-135° C.) at a line speed of 30 ft/min (9.1 m/min) to yield a peel strength of 5.7 g/in (0.2 g/mm) and between nip rollers at 128° C.-130° C. at a line speed of 40 ft/min (12.2 m/min) to yield a peel strength of 30 g/in (1.2 g/mm).
While the foregoing processes have produced commercially viable multi-layered, microporous films suitable for use as battery separators, there is a desire on the part of both the separator manufacturers and the battery manufacturers to have such films with greater interply adhesion (i.e., resistance to peeling individual layers from one another, measured by peel strength). One route, mentioned above, is to co-extrude the multi-layered film. From co-extrusion, an infinite peel strength may be obtained because the polymers at the interface of the layers are knitted together during extrusion. However, when individual layers are extruded and subsequently bonded (or laminated) together, peel strengths have been limited (as noted above).
Accordingly, there is a need to improve the peel strength of multi-layered microporous films made by laminating together precursors.